1. Field of the Invention
The invention relates to an image converting apparatus, more particularly to an image converting apparatus with a document contacting device that is capable of eliminating thin-film interference and to a method for eliminating thin-film interference in an image converting device.
2. Description of the Related Art
A conventional image converting apparatus, such as an optical scanner, photocopier or an image projector, has a document contacting device with a contact surface for contacting a document. The document contacting device is made of transparent glass when applied in an optical scanner or in a photocopier. In an image projector, the contact surface of the document contacting device is a reflective surface. The document may be a transparent projector film with an image thereon, a photograph or a sheet of paper. The optical scanner converts the image on a document into electrical signals for storage in a memory device or for display on a computer monitor. The photocopier converts the image on a document into an appropriate signal for duplicating the image on a sheet of paper. The image projector converts the image on a transparent projector film into a light image that can be displayed on a projector screen.
Usually, the resulting image generated by the image converting apparatus is not clear or has lost lines due to improper contact between the document and the document contacting device. It is noted that, in the conventional image converting apparatus, the contact surface of the document contacting device is a smooth and flat surface. Thus, when a document is made to contact the contact surface of the document contacting device, a thin layer of air is usually present between the document and the contact surface, thereby resulting in thin-film interference.
Thin-film interference is described in greater detail with reference to FIG. 1. As shown, when a light beam (a) passes through a thin layer 10 of air having a thickness (t) ranging from 150 nm to 1000 nm, two penetrating light beams (cd) and (cfhi) are generated. When the phase difference of the penetrating light beams, (cd) and (cfhi), is equal to half a wavelength or is an odd multiple of half the wavelength, an additive type of interference occurs. When the phase difference of the penetrating light beams, (cd) and (cfhi), is equal to an even multiple of half the wavelength, a subtractive type of interference occurs. In either case, a series of bright and dark bands, similar to Newton's rings, is generated. This is also true for the reflected light beams (be) and (bcfg).